1. Field of the Invention
The present invention relates to a process and an apparatus for press-forming strips.
2. Description of the Related Art
Known articles press-formed from strips are represented, for example, by a sheet metal having a shallow draw-formed brim on the periphery and a plurality of evenly spaced holes inside for supporting tubes in the heater core of an automobile heater.
FIG. 1 shows a known press forming process for producing the sheet metal by a "transfer press forming". In a press forming line composed by arranging a plurality of press machines each having a pair of die and punch, a sheet is trimmed in the first press machine to a blank with a required size and shape, the blank is then transferred to the second press machine for drawforming, further transferred to the third press machine for piercing holes through the sheet thickness, and finally transferred to the fourth machine for burring the holes to complete a product sheet metal.
Also known is "progressive press forming" as reviewed by Yasusada Yonemura in "Jitsurei Puresukako To Katasekkei No Kaizen (Practical Press Forming and Improved Die Designing)" published from Nikkan Kogyo Shimbun Sha (Daily Industry News), Feb. 28, 1969. A strip is fed through "progressive dies" arranged in series for several press forming steps in a single press machine, to continuously produce press-formed products with the same shape.
The transfer press forming process, however, has a problem that a minor change of design of a product, such as the length or other dimensions, necessitates changing dies for the respective press forming steps, increasing the cost of die preparation and requiring additional setup time for the die change.
The progressive press forming process also has a problem that the die arrangement including component dies for several press forming steps has a complicated structure, so that a change of design in the number of holes to be pierced or in the length of a product necessitates changing component dies as a whole, increasing the cost of die preparation and requiring additional setup time for the die change.